Plasma source

ABSTRACT

A plasma source is described. The source includes a reactive impedance element formed from a plurality of electrodes. By providing such a plurality of electrodes and powering adjacent electrodes out of phase with one another, it is possible to improve the characteristics of the plasma generated.

FIELD OF THE INVENTION

The present invention relates to a plasma source and in particular to aplasma source with reactive elements configured to be out of phase withone another so as to provide for controlled wavelength effects withinthe plasma process.

BACKGROUND

A plasma is an ionised gas that conducts electricity. In order togenerate a plasma, an electrical field is applied to a contained gas,usually contained within a specifically designed chamber. In a vacuumchamber, where ions and electrons have long lifetimes, it is relativelyeasy to do this. Radio frequency (RF) power in the MHz range can beapplied to two metal plates, or electrodes, immersed in the chamber,thereby creating a capacitive discharge. Alternatively, RF power may bedeposited into a coil mounted on the chamber walls, thereby producing aninductively coupled plasma.

In the semiconductor industry, plasmas are used to deposit materials onand etch materials from workpieces that are typically semiconductor,dielectric and metal surfaces. This process is utilised so as to formspecific electronic components on the substrate. A gas is introducedinto a vacuum plasma processing chamber where the workpiece is located.The gas by undergoing an electrical breakdown forms a plasma in anexcitation region using either an inductive source, where the antennacarries a current adjacent to the plasma window or a capacitive sourcewhich uses one (or more) electrode(s) with an oscillating voltage. Upuntil the early 1990's capacitive based systems were the preferredoption but in the time period 1991 to 1995, inductive sources becamemore prevalent, and they continue to dominate in metal etch or poly etchapplications. There are however problems with such inductive sourceplasmas in oxide etch applications. Furthermore, designs of inductivesystems for oxide etch that provide the necessary performance andstability for manufacturing criteria results in the cost of an inductivebased system being quite high.

Around 1998 the manufacturers of these systems, companies such as LamResearch Corporation and TEL started to refocus on capacitive systems soas to provide a cheaper and more reliable solution to the problems ofplasma etching in this field. Further developments led to thereintroduction of capacitive systems at the expense of inductivesystems. It is into this environment that dual frequency capacitivesystems re-emerged as the preferred choice for oxide etch applications.

The reason for this trend towards dual frequency systems is that in asingle frequency capacitive reactor, it is possible to increase the RFpower to get higher ion bombardment energy, but the plasma density willalso increase. These two parameters cannot be changed independentlyusing a single frequency generator. In order to provide an additionaldegree of flexibility, more than one frequency of excitation of acapacitive plasma can be provided. A typical approach, such as thatdescribed in WO03015123, employs two separate power supplies (a highfrequency supply and a low frequency supply), each attached to oneelectrode. Filtering is employed to minimize the interaction between thetwo signals, for example using an inductor that grounds the topelectrode at a KHz signal, while appearing to be a high impedance for aMHz signal. Similarly, a capacitor is used to ground the lower electrodefor high frequency signals. Alternative configurations include triode crconfined arrangements where the plasma is confined within a specificradial geometry and a further arrangement where both supplies areconnected to the same electrode can also be employed. In all cases thesubstrate, and therefore necessarily the associated substrate handlingcomponents such as pins and lifters, coolants, sensors etc., are RFdriven so coupling to the outside world needs to be sympathetic to thoseenvironments. This results in added engineering complexity—addinginevitably to cost.

To a fair approximation, in a dual frequency capacitive system the highfrequency power controls the plasma density; due to the higher currentsmore efficient displacement current increasing the ohmic power into theplasma and sheath heating mechanisms. The low frequency excitationinfluences the ion bombardment energy. Therefore, the user has someability to separately adjust the ion bombardment energy and the plasmadensity, which is not very easy with a single excitation frequency.Reactors of this design have found applications in both PECVD (plasmaenhanced chemical vapor deposition) and plasma etching.

Despite these advances in reactor design a number of problems stillexist. These include wavelength effects which introduce currents in theplasma parallel to the electrode surfaces, and under these conditionsthere is also non-uniform power deposition, which may be expected toproduce non-uniform plasma density which degrades the performance of theplasma.

There is therefore a need to provide a plasma source which is configuredto overcome these and other problems.

SUMMARY

These and other problems are addressed by a plasma source in accordancewith the invention. Such a source, according to a first embodiment ofthe invention provides a plurality of adjacent electrodes, eachelectrode being out of phase relative to its adjacent neighbour.

The electrodes can be configured in any one of a plurality of differentgeometrical configurations including for example planar, hemispheric,dome, convex, concave and/or undulating. The electrodes could beprovided so as to be in direct contact with the generated plasma. Usingan arrangement in accordance with the present invention it is possibleto control the relative centre to edge power deposition by modifyingelectrode spacings and/or power distribution design and/or the inclusionof active elements such as capacitors and/or inductors.

The invention therefore provides a plasma source according to claim 1with advantageous embodiments being detailed in the dependent claims.The invention also provides a method of operating a source.

These and other features of the invention will now be described withreference to exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic showing an illustrative embodiment of theinvention.

FIG. 2 is a modification to the system of FIG. 1 showing an alternativecoupling arrangement for the LF power supply.

FIG. 3 shows, in schematic form, an arrangement for using the plasmasource of the present invention with a roll of film.

FIG. 4 is an example of an electrode arrangement that can be used in atri-phase powering arrangement.

FIG. 5 shows a typical arrangement for the phase differences betweenthree supplies for use with the electrode arrangement of FIG. 4.

FIG. 6 shows an alternative pumping arrangement for introducing andpumping of gas in the plasma chamber according to the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of operational components of a plasma source100 in accordance with an embodiment of the present invention. Thesource 100 includes a plasma excitation region 110, into which a processgas may be introduced. This region defines the ultimate plasma volumeand it is within this region that the gas is converted into a plasma,which is then used to process workpieces placed within the region. Aplasma exciting reactive impendence element 105 is provided above theexcitation region 110. This element is coupled to a high frequency (HF)generator or source 125, the application of which to the element beingused to control density of the plasma. Within the present descriptionthe term high frequency is intended to encompass electromagneticradiation provided in the range 30 kHz-300 GHz, which sometimes would bereferred to as frequency in the radio-frequency to ultra-high frequencyrange. A reference electrode 115 is displaced below the region 110 andis optionally coupled to a low frequency (LF) source, the application ofwhich is used to control the energy of the ions striking the surface (asper present state-of-the-art). The reference electrode provides a mountfor the workpiece (not shown), which is typically a semiconductor,dielectric or metal substrate. The application of suitable fields to theelements 105, 115 serves to generate and maintain the correct ratio ofions and radicals relative to neutral species in the plasma and acontrol of the energy of the ions incident on the workpiece; gastransport and the residence time of these particles in the excitationregion play an important role. This control is required to ensure acorrect methodology for the selected deposition or etch processes beingemployed.

In accordance with the present invention, the reactive element isfabricated from a plurality of individual electrodes, shown in thisexample as four electrodes 105 a, 105 b, 105 c, 105 d, the fourelectrodes combining to form two sets of electrodes 105 a/105 c and 105b/105 d. Desirably, an even number of electrodes are provided and eachof the electrodes is individually coupled to the high frequency powersupply which is configured to provide a differential signal to adjacentelectrodes. In this manner the signal applied to a first electrode 105 ais out of phase with the signal applied to its immediate neighbour 105b. Similarly electrode 105 b is out of phase with electrode 105 c andelectrode 105 c is out of phase with electrode 105 d. In this way it canbe considered that the high frequency generator or drive creates adifferential between sets of electrodes. By the very nature of inductivecoupling, wavelength effects will be present in the electrodes and theplasma but the multiple electrodes that make up the reactive element ofthe present invention are advantageous in that the wavelengths effectscan be controlled so as to yield the desired plasma density as opposedto the traditional single electrode problem of non-uniform effects. Thedimensions of the individual electrodes are chosen and optimized suchthat non-uniformities on the scale-length of the electrode size thatoccur adjacent to the reactive elements do not result in excessiveplasma non-uniformity at the substrate. It will be appreciated thatthese dimensions may vary depending on the specific application forwhich the plasma source is used but desirably the size of each of theindividual electrodes is less than or equal to the distance between thesource and the substrate or workpiece and is such as to provide uniformeffects, if so desired for the particular application. A transformer 111may optionally also be included if there is a requirement for theequalisation of currents.

FIG. 2 shows a plasma source 200 which is a modification to thearrangement of FIG. 1 where both the LF and HF supplies are coupled tothe reactive element. In this embodiment, the HF generator and the LFgenerator may be applied simultaneously or independently of one another.By coupling both generators to the same reactive element plate it ispossible for the lower electrode, the reference electrode, to begrounded. It is not necessary to provide a capacitor in this path toground (i.e. the reference electrode can be directly coupled to ground),and this arrangement of enabling the reference electrode to be groundedis highly advantageous in that the engineering requirements for thechamber are simplified. For example, in arrangements where a moveablebottom stage was provided it was traditionally necessary for the bellowsthat make up that moveable stage to define an unknown and variableimpedance path; with the grounding of this lower stage, this is nolonger a requirement. It will be appreciated that using techniques knownin the art that the effect of the LF output can be maximised in theregion of interest by confining the plasma volume. This can be achievedin a variety of ways such as for example quartz confinement rings.

The LF supply can be provided in either a differential or a common mode.In a differential mode, with the low frequency signal applied to a firstelectrode being out of phase with that provided to its immediateneighbour, the ion energy is provided on the reactive element electrodesor on a dielectric material coupled thereto. If the LF supply isprovided in a common mode, then greater ion energy is provided on thereference electrode. This driving of the plurality of electrodes makingup the reactive element in a common mode configuration thereforecontrols the ion bombardment onto the workpiece that is mounted on thereference electrode. It will be understood that differential moderesults in lower ion energy to the substrate (reference electrode) butmaintains high ion energy to reactive elements for sputtering materialand/or keeping electrodes clean from deposition. Similarly to thatdescribed with reference to FIG. 1, a transformer 112 may optionally beprovided for coupling LF either in common mode or differential mode.Furthermore, the illustrated methodology used for providing LF will beappreciated as being exemplary of the type of methodology that could beimplemented as it will be appreciated that other techniques such as forexample, low pass filters or LF match-box components connecting LF powerto the HF lines, could be used to couple LF power into the system.

The generators or supplies can be operated in either VHF or RF modes,with the difference being that in VHF mode the high frequency willcouple inductively whereas in RF mode, it will be coupled capacitively.The ability to change frequency enables one to control the transfer froman inductive discharge to a capacitive discharge, so that one can gofrom high to low frequencies and vice versa without resultantnon-uniform etch (or whatever surface treatment is being provided usingthe plasma treatment) profiles resulting on the workpiece, as wouldhappen if a single electrode were utilised as in the prior artarrangements. Although the actual frequency at which the inductivedischarge becomes predominant is not exact, it is thought that atfrequencies of about 500 MHz, that the plasma discharge is predominatelyinductively based.

In a modification to that described heretofore, the present inventionalso provides for the HF source to operate in a switch mode as opposedto a sinusoidal operation. Such a switch mode operation is advantageousin that it is possible to alter the slew-rate of the switch region so asto yield an “effective frequency” which will determine amount ofinductive coupling. The length of time the reactive element is left inthe high voltage state would control the ion bombardment energy. Switchmode generators are very well known in the general electronics fieldwith well defined characteristics and components. The ability to usesuch a switch mode generator provides for a reduction in cost of theplasma source—switch mode generators are cheaper than the equivalentsinusoidal based generator. By controlling the slew rate one is able tomove easily from the RF range to Ultra High Frequencies (UHF) therebyproviding the possibility to tune the process chemistry and/or theelectron temperature, T_(e).

Heretofore, the invention has been described with reference to a plasmasource configured to operate with a planar workpiece, where theelectrodes making up the reactive impedance element and the referenceelectrodes are substantially parallel to each other and to theworkpiece. Such arrangements are advantageous and useful for applicationin the semiconductor environment where a planar wafer is provided foretching. However it is known that plasma sources can also be used inother applications where it is desired to process a non-planarsubstrate, for example a roll of film in a textile screen printingapplication. FIG. 3 shows in schematic form how the present inventioncan be configured for use in such an arrangement 300 where a roll offilm 305 is originally provided on a reel 310. The film is unwound fromthe initial reel 310 on an unwind station 315, passes through the plasmasource 105 where it is processed and is re-wound on a rewind station320. The plasma source of the present invention is suitable forprocessing such large dimension surfaces because the multiple electrodesthat make up the reactive element enable the provision of uniform plasmaover an extended area. The arrangement of the present invention allowsfor the provision of higher frequency sources to be used and thereforethe speed of the film through the plasma source can be increased. Thesehigher frequencies do not lead to a detraction in the quality of theplasma as the multiple electrodes of the reactive element provide forhigher density application without detracting from the uniformity of theapplied plasma. It will be appreciated that such an arrangement may alsobe modified for plasma screens, LCD displays, industrial coating onmetal/glass, and the like where simultaneous processing of large areasis required. Although the LF supply shown in this embodiment is coupledto the substrate plate it will be understood that in a manner similar tothat described with reference to FIG. 2 that one could also have a lowfrequency feed through the upper reactive elements.

Certain applications may require the use of a curved processing area.The present invention provides for such processing in one of two ways.In the first, in a manner similar to that described with reference toFIG. 3, the invention utilises a substantially planar arrangement ofreactive elements to process a curved workpiece. FIG. 4 shows analternative arrangement in accordance with the teachings of the presentinvention, where the source could be applied to non-planar plasmavolumes. In this example it may be advantageous to provide an electrodeconfiguration that can be configured in a non-planar configuration, andthis example uses a hexagonal close pack configuration 400 including aplurality of individual hexagonally dimensioned electrodes 405. In thisexample, a 3 phase drive mechanism is used as opposed to the direct pushpull operation of the configuration of FIGS. 1 to 3, and each of theindividual electrodes is coupled to a respective one of the threesources (identified by the labelling 1, 2, 3 respectively). As with theembodiment of the previous Figures, no two adjacent electrodes are inphase with one another—see FIG. 5 for an example of the outputconfiguration for each of the sources. To assist in current imbalance atri-filar transformer may be used which is advantageous in that itallows for the provision of a low voltage above the substrate and anequalisation of the currents. In other circumstance, it will beappreciated that certain applications where it is useful to drive a netcurrent and therefore a net voltage intro the reference electrode mayfind it advantageous to have a current imbalance. It will be furtherappreciated that the examples of two and three phase sources areexemplary of the type of frequency generator that could be used with thereactive elements of the present invention and that certain otherapplications may require sources capable of providing a higher orderphase supply.

Although the plasma source of the present invention may be used withknown gas distribution feeds such as a shower effect electrode withradial gas flow and pumping on the perimeter of the plasma volume, thepresent invention also provides in certain embodiments sources thatutilise a gas distribution feed that enables the removal of gas awayfrom the lower reference electrode. FIG. 6 shows a portion of such asource where two adjacent electrodes that make up the reactive elementare illustrated. The electrodes are mounted below a gas feed chamber600, and the gas that is within this chamber may be introduced firstlyto a feed chamber 630 through an entrance conduit 620 and then into theplasma excitation region 110 through a plurality of apertures 605provided in the electrodes of the type that will be known to thoseskilled in the art from existing showerhead technology. Once the gas hasentered into the excitation region 110 it then flows towards a groundplate 610 providing a gas exit 615 above the electrodes into a pumpingplenum 620. The pumping plenum is electrically isolated from the plasmavolume thereby obviating the possibility of the plasma reforming in thisregion. This pumping of the gas away from the excitation region obviatesthe possibility of the etchant gas interacting with the substrate beingtreated on the reference electrode. It may be necessary in thisarrangement where the gas is moved around the electrodes to coat theelectrodes with a dielectric material 625 such as silicon dioxide or thelike. Such a dielectric coating is shown as defining the exit path forthe gas, but the exact extent of the coating could vary depending onapplication.

It will be appreciated that what has been described herein is a newplasma source which provides for centre-to-edge power deposition byelectrode spacing and/or power distribution design and/or activeelements such as capacitors and/or inductors so as to provide for acontrolled uniformity profile plasma. In certain applications this canrequire a difference in the profile of the plasma at certain regions ofapplications, such that specific selected areas are regions of greaterplasma deposition as opposed to other. Other applications may requirethe same profile across the substrate. Although, the electrodes havebeen described with regard to exemplary embodiments it will beappreciated that the configuration chosen for a specific application canbe such so as to have the electrodes arranged on any arbitrary—shapeplasma facing element including for example flat, hemispheric, dome,convex, concave, undulating. The electrodes could be in direct contactwith the plasma or could alternatively interact with the plasma througha dielectric window provided from materials such as SiN, AlN, SiC, SiO₂,Si etc. The arrangement of the present invention provides a number ofdistinct advantages over the prior art including:

-   -   1. Compatible with HF+LF independent control of ion energy        (E_(ion)) and ion flux (Γ_(ion)).    -   2. Controllable T_(e) is a possibility by allowing for a HF        scanning from RF to UHF.    -   3. As the individual electrodes making up the reactive element        may be dimensioned small, and the dimensions of these can define        the plasma volume it is possible to provide a plasma source        having a small plasma volume. Any individual non-uniform power        coupling from an individual electrode or pair of electrodes does        not result in non-uniform plasma density at a large enough        distance from the electrodes. Specifically, it will be        understood that as each of the individual elements are reduced        in size that the distance required within the plasma volume for        the overall generated plasma to be equalised is reduced.    -   4. The source may be used with substrates of many different        dimensions as it may be configured to provide minimal        centre-to-edge power deposition effects over an extended area        and as such is suitable for large substrates (300 mm wafers,        FPD, textiles and the like).    -   5. Similarly, the possibility of using high frequency sources is        advantageous as one can choose the operational frequency to        match the process required, and it is possible to go to higher        frequencies than heretofore achievable without introducing        plasma non-uniformity.    -   6. The source may be used with gas distribution feeds similar to        present generation systems or alternatively may be used with a        distribution feed that minimizes any interaction between etchant        or deposition by-product gas and the substrate material.    -   7. Reduced system cost as lower electrode can be grounded. This        is particularly advantageous in that there is no longer a        requirement to provide a high frequency lower plate, which had        the requirement that ancillary equipment needed to be isolated        from ground, whereas the configuration of the invention enables        the ancillary equipment to be grounded.    -   8. Compatible with advanced HF power supply technology and        direct-drive switch-mode power, which can provide the necessary        frequencies at a lower cost. No HF through lower electrode, so        variable gap easier to engineer. As the HF component is applied        to the reactive elements solely it is possible to minimize the        HF return through chamber body, so unconfined plasma should be        less likely to occur. Furthermore, there is no longer the        requirement to stringently provide for such HF paths in other        components of the chamber.

It will be understood that the invention provides for plurality ofphysically individually distinct reactive elements with adjacentelectrodes being coupled out of phase with one another. It will beappreciated that if two adjacent electrodes are couple in phase with oneanother that they in effect resemble a physically larger singleelectrode, and that such a single larger electrode will be out of phasewith its immediate neighbours.

The reactive elements of the invention may be provided in anyconfiguration or array structure, for example a 2-D array or linearstructure which may, it will be appreciated, be scaled in dimensiondepending on the application required. It will be appreciated that theconfiguration of the present invention provides for such scaling whilemaintaining compatibility with VHF/UHF operation requirements andperformance levels.

Therefore although the invention has been described with reference toexemplary illustrative embodiments it will be appreciated that specificcomponents or configurations described with reference to one figure mayequally be used where appropriate with the configuration of anotherfigure. Any description of these examples of the implementation of theinvention are not intended to limit the invention in any way asmodifications or alterations can and may be made without departing fromthe spirit or scope of the invention. It will be understood that theinvention is not to be limited in any way except as may be deemednecessary in the light of the appended claims.

Similarly, the words comprises/comprising when used in thisspecification are to specify the presence of stated features, integers,steps or components but does not preclude the presence or addition ofone or more other features, integers, steps, components or groupsthereof.

1. A plasma source comprising: a plasma excitation region; a groundedreference electrode positioned in a first side of the plasma excitationregion; a plasma exciting reactive impedance element having a pluralityof electrodes arranged side-by-side in a second side of the plasmaexcitation region; a high frequency generator coupled to the pluralityof electrodes of the plasma exciting reactive impedance element andconfigured to apply out-of-phase signals to adjacent electrodes of theplurality of electrodes of the plasma exciting reactive impedanceelement to control wavelength effects of high operational frequencies;and a low frequency generator coupled to the plurality of electrodes ofthe plasma exciting reactive impedance element and having a differentialoperational mode and a common operational mode, wherein the highfrequency generator and the low frequency generator are configured toindependently control process parameters of plasma density and ionenergy, respectively.
 2. The source as claimed in claim 1 wherein thereactive element is provided adjacent to the plasma excitation region.3. The source as claimed in claim 1 wherein the high frequency generatoris tunable from frequencies in a radio frequency range to frequencies ina ultra high frequency range.
 4. The source as claimed in claim 1wherein the low frequency and high frequency generators are operablesimultaneously.
 5. The source as claimed in claim 1 wherein each of thelow and high frequency generators are configured such that they can heindividually applied so as to provide a desired process output.
 6. Thesource as claimed in claim 1 being further configured to support aworkpiece.
 7. The source as claimed in claim 6 wherein the workpiece ismountable adjacent the reference electrode.
 8. The source as claimed inclaim 6 being configured to enable a movement of the workpiece throughthe plasma excitation region.
 9. The source as claimed in claim 1wherein the plurality of electrodes are provided in a planar arrangementindividual electrodes of the reactive impedance element being axiallyaligned with others of the reactive impedance element.
 10. The source asclaimed in claim 1 wherein the plurality of electrodes are arranged toprovide a curved element.
 11. The source as claimed in claim 10 whereinthe curved element is used to enable a processing of non-planarworkpieces.
 12. The source as claimed in claim 10 wherein the curvedelement is used to process a planar workpiece.
 13. The source as claimedin claim 1 wherein the reactive element is configured to enable a gasfeed through selected ones of the plurality of electrodes.
 14. Thesource as claimed in claim 13 wherein the reactive elements isconfigured to enable a gas feed through all of the electrodes.
 15. Thesource as claimed in claim 13 wherein individual electrodes of thereactive element are configured in a showerhead configuration.
 16. Thesource as claimed in claim 1 further including a pump, the pump enablinga pumping between selected adjacent electrodes of the reactive elementso as to provide for a removal of gas from the plasma excitation region.17. The source as claimed in claim 16 being configured to enable aremoval of the gas to a region of the source which is electricallyisolated from the plasma excitation region thereby preventing thereforming of a plasma.
 18. The source as claimed in claim 1 wherein thehigh frequency generator comprises a dual-phase supply.
 19. The sourceas claimed in claim 1 wherein the high frequency generator comprises atri-phase supply.
 20. The source as claimed in claim 1 wherein the highfrequency generator has a switch mode configuration.
 21. The source asclaimed in claim 1 wherein selected ones of the plurality of elementsare provided with an outer coating selected from a non-conductive ordielectric material.
 22. A method of operating a plasma sourcecomprising a plasma excitation region, the method comprising: applying afirst high-frequency signal from a high-frequency signal generator to afirst electrode of a plasma exciting reactive impedance element, thefirst high-frequency signal having a first phase; applying a secondhigh-frequency signal from the high-frequency generator to a secondelectrode of the plasma exciting reactive impedance element, the secondelectrode being adjacent to the first electrode and the secondhigh-frequency signal having a second phase different from the firstphase; and tuning the high-frequency generator to obtain a desiredprocess output.
 23. A method of operating a plasma source comprising aplasma excitation region, the method comprising: selectively applying afirst high-frequency signal from a high-frequency signal generator to afirst electrode of a plurality of electrodes of a plasma excitingreactive impedance element, the first high-frequency signal having afirst phase; selectively applying a second high-frequency signal fromthe high-frequency generator to a second electrode of the plurality ofelectrodes of the plasma exciting reactive impedance element, the secondelectrode being adjacent to the first electrode and the secondhigh-frequency signal having a second phase different from the firstphase; and selectively applying a low-frequency signal to at least oneelectrode in the plurality of electrodes of the plasma exciting reactiveimpedance element.
 24. The method of claim 23, further comprising:adjusting the high frequency signals to control process parameters ofplasma density; and adjusting the local frequency signal to controlprocess parameters of ion energy.